To switch operation to four-cycle operation even when arranging a piston and suction-exhaust valves so as to positionally interfere with each other for realizing the high compression ratio in response to two-cycle operation by self-ignition combustion.

For providing the high compression ratio required for performing premix compression self-ignition two-cycle operation, the piston 144 of a reciprocating engine 10 and the suction-exhaust valves 132 and 134 are arranged so as to cause positionally overlapping interference in the vicinity of the top dead center TDC of the piston. The opening-closing valve timing of the suction-exhaust valves is set so that the suction-exhaust valves 132 and 134 do not interfere with the piston 144 in the vicinity of the top dead center TDC when switching this engine 10 to the four-cycle operation. As a result, the operation can be switched to the four-cycle operation from the two-cycle operation by the high compression ratio without arranging a mechanism for varying a stroke position of the piston according to an operation cycle.